When I look at the state of things after a couple of years out of the front line, I now very much tend towards a more simplified schema. Especially with so many distros out there, and also the relatively short time interval between Kanotix distros.

Is there anything to stop me from treating each distro iso as just a separate r/o program, and the boot loader as just a loader that loads and runs these different 'programs'?

There doessn't seem to be any point in installing to disk when I can make do with the persistence facility. One just has to reserve enough h/d space for the iso image and the corresponding persistence file, while the rest of h/d storage can be turned over to data. So no more update headache. And if one distro is missing a functionality, there's bound to be another out there with the sought after functionality.